This invention relates to electrical batteries, and particularly to a novel lithium battery of one or more cells.
Lithium batteries have been long known to have potentially superior qualities, such as tolerance for low temperature, high energy density, and long shelf life, but have hitherto been adopted for use only in limited quantities and for highly specialized applications. As pointed out in an article entitled "Focus On Lithium Batteries: High Energy For Critical Needs", Electronic Design, Dec. 10, 1981, pp. 183-198, at page 183, the reasons for this limited acceptance have been simple, but compelling; conventional lithium batteries are very expensive, and have a reputation for being dangerous.
The conventional approach to the design and construction of lithium batteries revolves about a conception of the battery as a single cell encapsulated in a metal container, as of nickel plated steel, and comprising a lithium disc formed integral with a grid of nickel plated steel, or stainless steel, as a current collector. The cathode is a solid pressed pellet of any of various reductants held in a metal cup, as by a metal ring. Such a construction is described, for example, in U.S. Pat. No. 3,853,627. This arrangement has manifest advantages in protecting the cell from mechanical damage; in preventing interactions between potentially reactive components of the atmosphere, such as O.sub.2, N.sub.2, CO.sub.2, H.sub.2 and H.sub.2 O, and the chemically active cell components; and in ameliorating to some degree the consequences of pressure generated within the cell, either by volatile electrochemical constituents such as SO.sub.2, or by gases formed in secondary reactions between cell components and contaminants that are formed during storage or are not adequately excluded or removed during cell manufacture. However, the provisions made in this way to ensure cell integrity have significant attendant disadvantages; notable among these are the complexity and consequent cost of the construction, and the potential for explosion caused by excessive internal gassing, internal contaminants, misuse or improper disposal techniques. The potential for explosion is inherent in the use of the rigid metal cell container; i.e., in the very expedient adopted to minimize the probability of potentially explosive reactions.
Recent advances in lithium batteries are described in the following copending U.S. Applications for Letters Patent, filed on Jan. 20, 1982, by Alan H. Bruder and assigned to the assignee of this invention, all of which are hereby incorporated herein by reference in their entireties:
U.S. application Ser. No. 340,990, for Hermetically Sealed Lithium Batteries; U.S. application Ser. No. 341,195 for Lithium Batteries With Organic Slurry Cathodes; U.S. application Ser. No. 341,196 for Laminar Multicell Lithium Batteries; and U.S. application Ser. No. 341,197 for Lithium Batteries With Laminar Anodes.
The objects of this invention are to simplify the construction and improve the reliability of lithium batteries.
The use of organic slurry cathodes, as described in the above cited U.S. application Ser. No. 341,195, has been found to alleviate many of the problems encountered with pressed bound cathode pellets, the conventional alternative. Basically, the organic slurry cathode comprises a slurry of MnO.sub.2 and carbon in a solution of a lithium salt in an organic solvent. In order to obtain the requisite solubility and ionic activity for the lithium salts used as electrolytes in lithium systems, organic solvents of considerable solvent power, such as propylene carbonate, 1,2 dimethoxyethane, and the like, are conventionally employed. As noted in the above cited U.S. application Ser. No. 341,196, such solvents appear to have the ability to permeate the conductive plastic intercell connectors used to isolate the active cell components electrochemically while providing a low impedance path for electronic conduction. This permeation is not sufficiently marked to be characterized by any appreciable solvation or swelling of the conductive plastic, and has in fact been deduced to occur only from the phenomenon of an impedance buildup in batteries in which a conductive plastic boundary was employed to separate an organic slurry cathode from a lithium anode. As described in the above cited application Ser. No. 341,196, this tendency can be prevented by sandwiching a metal solvent barrier between layers of conductive plastic, to form a solvent impermeable intercell connector. However, it has now been found that over a period of time at normal temperatures, or much more rapidly at elevated temperatures in the neighborhood of 120.degree. F., the solvents used in the organic slurry cathode show a tendency to weaken or destroy the adhesive bond between conductive plastic laminae, in contact with the slurry and used for electrochemical isolation, and adjacent metal foil laminae used either as solvent barriers or, for their electrical conductivity and mechanical strength, as external electrical terminals.
There are great practical difficulties involved in the resolution of the problem posed by an unfavorable interaction between the organic solvent employed in the cathode slurry and the conductive plastic-metal interface. At this interface, a conductive plastic adhesive is employed, because an adequate degree of adhesion between the conductive plastic and the metal foil, of aluminum, tinned steel or the like, cannot otherwise be attained. The desired degree of adhesion is attained by including small amounts of reactive functional groups in the adhesive to promote adhesion to the metal foil layer. These groups are speculated to be selectively affected by the potent organic solvent.
The alternative, of eliminating the conductive plastic and placing the cathode slurry directly in contact with the metal foil terminal, is only superficially attractive because the various functions well performed by the conductive plastic are antithetic to the normal metallic nature. In particular, the conductive plastic is relied upon for adhesion to the cathode slurry sufficient to provide a low impedance electronic bond; for adhesion to adjacent thermoplastic components sufficient to provide a liquid impermeable seal around the periphery of the wet active cell components; and to provide electrochemical isolation between the electrochemically active components of the cells and external metallic components or adjacent active cell components. Most of the metals fail in one or all of these desired attributes, and only such exotic and inordinately expensive metals as gold, silver and platinum are sufficiently inert electrochemically to warrant the appellation "inert". Some of the stainless steels are sufficiently inert to warrant consideration from that standpoint alone, but lack adequate adhesive properties and are too costly for use in most applications. Tinned steel, while readily available and sealable, is not sufficiently inert, tending to form destructive couples with the MnO.sub.2 in the cathode. Such metals as aluminum and zinc are far too active for consideration.
In accordance with this invention, it has been found that copper, while too active for use in a conventional lithium-MnO.sub.2 cell, can be employed as the substrate in contact with an organic slurry cathode comprising MnO.sub.2 if the assembled cell is subjected to a preliminary discharge, in the manner and for the purpose described in the above cited application Ser. No. 340,990, soon after electrochemical assembly; i.e., within a few minutes or hours after electrochemical assembly of the cell. Preferably, the copper used as the substrate in contact with the slurry in accordance with the invention is extended out into the seal area surrounding the active components of the cell or cells, to preclude or inhibit the migration of the organic solvent in the cathode slurry into regions in which it might cause problems. In addition, the copper employed is preferably a copper foil having one or preferably both surfaces treated by conventional techniques to promote adhesion to adjacent substrates, as in the manners described in U.S. Pat. Nos. 3,220,897; 3,293,109; and 3,699,018; for example.
Lithium anodes for use with cathode slurries on copper substrates in accordance with the invention may be of conventional construction. Alternatively, in accordance with a novel aspect of the invention, preferred anodes may be constructed by laminating lithium directly to a surface of a copper foil treated to promote adhesion, with the result that a laminar battery containing no conductive plastic components may be constructed.
The practice of the invention will best be understood in the light of the following description, together with the accompanying drawings, of various illustrative embodiments thereof.